When light propagates at the interface between a metal and a dielectric such as glass, it interacts strongly with the collective electron oscillations at the metal surface. The resulting mixture, called a surface plasmon polariton (SPP),1 has a wavelength shorter than that of the incident light. Confining electromagnetic waves to scales much smaller than the free-space wavelength makes it possible to shrink the size of an optical system and increase the intensity of the electric field. SPPs therefore allow optical experiments to be performed at unprecedented subwavelength scales. This is contrary to dielectric-only structures such as optical fibers, where shrinking device dimensions results in less confinement due to diffraction.

Here we describe work in quantum plasmonics focused on creating, propagating, and detecting single SPPs. The main goal is to build integrated experiments aimed at quantum information processing, using an SPP as a data bit.

Superconducting nanowires that become resistive on absorbing a photon can be used as single-photon detectors.2 They have useful properties such as short dead time following a detection event, low dark-count rates, a large operating wavelength range (UV–IR), and straightforward operation. We have recently shown that these detectors can also be used for direct on-chip detection of single SPPs,3 making them suitable for integrated quantum optics experiments. In our experiment, we used a quantum dot as a single photon source and coupled it to a plasmonic waveguide: a narrow gold stripe deposited on a dielectric. The photons propagate along the gold-dielectric interface as SPPs and make their way to the superconducting detector. The sensitivity of this detector in the wavelength range >1μm is especially interesting for telecom applications since SPPs propagate longer with increasing wavelengths, which would ensure low losses.

As opposed to first converting SPPs into photons, this direct on-chip detection is efficient and enables the development of complex optics circuits at a small length scale. In the future, a single emitter such as a quantum dot could be integrated in close proximity to the waveguide. This would result in more efficient coupling of the emission and a potentially enhanced emission rate due to the Purcell effect.

We are currently building more advanced systems where plasmonic beam splitters and pairs of detectors are combined to form an interferometer: see Figure 1. Figure 2 shows experimental results. An experiment that is now within reach is the observation of Hong-Ou-Mandel interference with SPPs. This is a well-known effect in quantum optics that has only been observed with photons.4 In it, two indistinguishable photons arriving at a 50/50 beam splitter at exactly the same time always appear at the same output port. It is a purely quantum effect with no classical explanation, and can be used to realize linear optics quantum-computing gates.5 Demonstrating this type of interference with SPPs will prove their quantum nature and will pave the way to complex quantum-optical experiments at subwavelength scales.

Figure 1. A scanning electron microscope image of a plasmonic beam splitter circuit made of silver integrated with two superconducting detectors. The splitting ratio is controlled by the coupling length of the two waveguides (100nm separation).

Figure 2. Response of the left (a) and right (b) detector when scanning a laser spot across the device shown in Figure 1. Both inputs of the plasmon beam splitter clearly couple to both outputs, as they are visible in the signal from both the left and right detectors. The inputs do not appear equally intense because the in-coupling is polarization-dependent and the beam splitter has a 70/30 splitting ratio.